A laminated glass including two glass sheets and an interlayer film for laminated glass that contains plasticized polyvinyl butyral and is sandwiched between the glass sheets, all bonded together, has been widely used as window glass of automobiles, aircraft, architectures, and the like.
Interlayer films for laminated glass may include a single resin layer, or a laminate of two or more resin layers. If an interlayer film for laminated glass includes two or more resin layers that include a first surface layer and a second surface layer and have different characteristics, the interlayer film can provide various properties that are difficult to achieve with an interlayer film of a single layer structure.
For example, Patent Literature 1 discloses an interlayer film for laminated glass having a triple layer structure that includes a sound insulation layer and two protective layers sandwiching the sound insulation layer. The sound insulation layer of the interlayer film for laminated glass of Patent Literature 1 contains a polyvinyl acetal resin that is highly compatible with a plasticizer and a large amount of plasticizer, thus providing excellent sound insulation properties. The protective layers prevent the large amount of plasticizer in the sound insulation layer from bleeding out, thus preventing reduction of adhesion between the interlayer film and glass.
However, a laminated glass including such an interlayer film composed of a laminate of two or more resin layers may unfortunately cause optical distortion. Such optical distortion is pronounced especially in the case of a laminated glass including an interlayer film extended with heat to follow the curved surface of glass sheets (e.g., curved surface of windshields of automobiles). The optical distortion of laminated glass refers to a phenomenon that an object observed through the laminated glass looks distorted.